How to get webContents of an iframe via IPC?

I want to get the webContents from an iframe, that is located inside my index.html. Unfortunately my following approach doesn't work. What am I doing wrong?

I created an index.js containing:

const ipcRenderer = require('electron');

const iframe = document.getElementById('iframe'); // get iframe by id

let contents = iframe.webContents

contents.addEventListener('did-navigate-in-page', (event, input) => { // If navigation in frame occurs
    ipcRenderer.send('did-navigate-in-iframe', input); // Send event and navigation input
  })

which I required inside the index.html close to the iframe:

<main class="main_window">
    <script>
        require('./index.js');
    </script>
  <iframe  id="iframe" src="top.html" ></iframe>
</main>

In my main.js I am trying to

const ipcMain = require('electron')

ipcMain.on('did-navigate-in-iframe', (event, input) => {
    console.log(input) // Trying to log input here, which should be the location of the HTML file or link
});